Monitoring of gamma radiation is important for many applications, including nuclear power plants, medical radiology, and homeland security. Gamma radiation is particularly important to monitor and control because of its potential danger to the human body. There are two types of radiation which differ in their interaction with common chemical matter: ionizing and non-ionizing radiation. Ionizing radiation is considered more harmful because of its potential damage to DNA. Gamma radiation (also known as gamma rays) is a typical ionizing radiation (high energy photons, at a level of millions eV per photon), and research shows that the repair process for low dose gamma radiation exposure is much slower than for high dose exposure. Furthermore, the penetration depth of gamma radiation is much longer than α (42He) and β (electron, e) particles. This means that it is more difficult to shield gamma radiation than α (42He) and β (e) particles.
Gamma radiation is widely used in medical radiological therapy, the construction industry, and scientific research. For example, the medical treatment of tumors by gamma radiation (also referred to as “gamma knife”) requires precise adjustment or calibration of the equipment on a daily basis to ensure that it emits the proper level, or dose, of gamma radiation. The tolerance of gamma radiation intensity levels in this type of application can be as low as 2%.
Scientists and engineers have developed several technologies to measure the dose of radiation, such as ion chambers, scintillation detectors, and semiconductor detectors. The detection mechanisms for ion chambers and semiconductor detectors are similar and based on the monitoring of ion pairs generated by the ionizing radiation. When the particles (α or β) or photons pass the sensor materials, charges (electron/hole pairs) will be generated due to the interaction of the particles with sensor materials. The collected current is then used as a signal output. Typically, one particle will generate ˜30000 charged pairs and thus has a signal amplification effect. However, this mechanism suffers from low sensitivity in gas-based detectors and/or thermal noise at room temperature in semiconductor detectors. The scintillation detectors are based on fluorescence of the sensory material which is excited by radiation. But inorganic scintillation materials are based on single crystal material, which are costly to scale up. On the other hand, many previous organic scintillators do not have the energy resolution required for identification or quantification. Furthermore, the photon detectors used to detect light from the scintillator can suffer from external interference. For example, photomultiplier tubes (PMT) are sensitive to external magnetic fields, and silicon-based photodiodes are sensitive to the incident radiation itself. As such, accuracy of such devices can be unreliable.
Chemical sensors can provide advantages over the current instrument-based sensor systems, including ease of use, low cost, high adaptability for size miniaturization and combination or integration into current electronic instruments, high flexibility or conformability to be fabricated into various shapes or composite materials that are suited for different applications of radiation detection (e.g., nuclear security vs. radiology calibration in clinics), and unlimited options of molecular design and engineering so as to improve sensitivity. Although many types of chemical sensors have been developed, such chemical systems can only detect gamma radiation down a level of near 10 Gy, still several orders of magnitude higher than the level required for practical use in both nuclear security and medical areas. There is thus a technical gap between the research and development of chemical sensor systems and the real application of such systems.